This is known as thermal pollution or thermal discharge, and it is the introduction of waste heat into bodies of water that support aquatic life. The addition of heat reduces the water's ability to hold dissolved gases, including the oxygen required for aquatic life. If the water temperature is greater than 95°F, the dissolved oxygen content may be too low to support some species. If the differential temperature is too large, the difference can also stress some species.
As a result, thermal pollution can wreak havoc on native fish species, such as trout, that require cold water with high levels of dissolved oxygen. When the water becomes warmer, other non-native fish that thrive in the warmth can take over habitats from native fish. In addition, warmer water allows bacterial populations to increase and thrive, and algae "blooms" may occur.